A wireless LAN (local area network) in the 5 GHZ band is premised on the use of an OFDM (orthogonal frequency division multiplexing) modulation system. The OFDM modulation system processes received signals as described below. A received OFDM signal is sampled as a digital signal (data) in an AD converter. In the sampled data, a frequency component of a channel adjacent to the data is removed in a low pass filter by digital signal processing. Then, the data is subjected to extraction processing at a required sampling rate in a rate converter. The data subjected to extraction processing is subjected to Fourier conversion in an OFDM signal decoding circuit. Then, errors of the data converted in a Viterbi decoder is corrected, and thus the received OFDM signal is decoded.
In the OFDM modulation system, since the signal of the adjacent channel is removed in a digital filter (low pass filter), data of a broader band than an effective frequency bandwidth must be sampled (oversampled), and data at a high sampling rate is required.
Moreover, since the OFDM signal is obtained by superimposing a plurality of subcarrier signals, a ratio of the maximum amplitude and an average amplitude of the signal are increased in comparison with those of a phase shift keying (PSK) signal and a frequency shift keying (FSK) signal. Hence, sampling data to a base band LSI for executing receiving processing by use of the OFDM signal must have a large number of bits.
As described above, in the system for receiving an OFDM signal, for which a digital signal processing technology is used, a multi-bit and high-rate (high sampling rate) AD converter is required. However, since power consumption is increased in such an AD converter, this AD converter is not suitable for a mobile communication apparatus.
Moreover, in the OFDM modulation system used for the wireless LAN, a transmission rate of the data is selected in accordance with a condition of a transmission path, and accordingly, a modulation system of the subcarrier is changed. Therefore, in the receiving processing, the sampling rate and the number of sampling bits, which are required, are changed in some steps in response to the modulation system of the received OFDM signal.
For example, in the case of a data transmission rate of 54 Mbps, a 64 QAM (quadrature amplitude modulation) modulation system is adopted. However, accompanied with lowering the data transmission rate, the modulation system is changed to a 16 QAM and QPSK (quadrature phase shift keying), and in the case of the lowest data transmission rate of 6 Mbps, the modulation system becomes a BPSK (binary phase shift keying) modulation system. In the case of the BPSK modulation system, about 8 bits are conceived to be sufficient for data in the AD converter and after. Moreover, also in decision in the Viterbi decoder, even if the number of bits for soft decision is reduced, or even by 1-bit hard decision, it is conceived to be possible that a sufficient performance is obtained.
However, heretofore, a circuit has been designed and implemented in accordance with a specification in the case where the sampling rate and the number of sampling bits are the maximum. Hitherto, facilitation of the design and the implementation has been given priority, and a circuit suitable for the case of 54 Mbps where the data transmission rate is the maximums been employed. For example, the specification has been set such as; 12-bit/40 MSPS in the AD converter; 12-bit/20 MSPS in an FFT (fast Fourier transform); and 6-bit soft decision in the Viterbi decoder.
Therefore, even if the data transmission rate is lowered to change the modulation system, the power consumed in the AD converter or the circuit that executes the digital signal processing has not been able to be reduced.